Here's why:
* It modifies a verb, adjective, or another adverb. It tells us about the timing of something else in the sentence.
* It indicates a contrast or a change in events.
* It can often be replaced by other adverbs like "in the meantime" or "at the same time."
Example:
* Meanwhile, the sun was setting. (modifies the verb "was setting")
Let me know if you'd like more examples!